Wireless networks, in which a user equipment (UE) such as a mobile handset communicates via wireless links to a network of base stations or other wireless access points connected to a telecommunications network, have undergone rapid development through a number of generations of radio access technology. The initial deployment of systems using analogue modulation has been superseded by second generation (2G) digital systems such as GSM (Global System for Mobile communications), typically using GERA (GSM Enhanced Data rates for GSM Evolution Radio Access) radio access technology, and these systems have themselves been replaced by or augmented by third generation (3G) digital systems such as UMTS (Universal Mobile Telecommunications System), using the UTRA (Universal Terrestrial Radio Access) radio access technology. Third generation standards provide for a greater throughput of data than is provided by second generation systems; this trend is continued with the proposals by the Third Generation Partnership Project (3GPP) of the Long Term Evolution (LTE) system, using E-UTRA (Evolved UTRA) radio access technology, which offers potentially greater capacity and additional features compared with the previous standards.
Note that the term “GERA” is used herein to refer to the radio access technology associated with GERAN (GERA networks), “UTRA” is used to refer to the radio access technology associated with UTRAN (UTRA networks), and similarly the term “E-UTRA” or “LTE” is used to refer to the radio access technology associated with E-UTRAN (E-UTRA networks).
LTE is designed primarily as a high speed packet switched network, and voice services, packet switched voice services and in particular Voice over Internet Protocol Multimedia Subsystem (VoIMS) services are envisaged, whereas previous generation systems such as UMTS support voice services that are primarily circuit switched.
As new technology is introduced, networks are typically deployed which include radio access networks that use a radio access technology according to a recent standard and also legacy radio access networks that use a legacy radio access technology. A user equipment may be typically capable of communications using two or more radio access technologies, so for example the user equipment is able operate using one radio access technology, perhaps offering high capacity, where this is available, but being able to operate using a legacy radio access technology, in those service areas of the network that do not support the other radio access technology, or that do not support preferred features.
In service areas where a radio access network, such as an LTE/E-UTRA network, does not support voice communications, user equipment may follow a defined procedure to fall back to using another radio access network, such as UTRAN or GERAN, for voice communications, typically falling back to circuit switched voice communications.
There are a variety of handover procedures that have been developed to allow handover of user equipment between a E-UTRA network and a UTRA/GERA network. In particular, handover procedures have been developed to allow handover when a voice call is in progress, that is to say so-called Voice Call Continuity (VCC) handover procedures. Typically a VCC procedure will be implemented under the control of a session transfer controller, which will typically comprise a Service Centralisation and Continuity Application Server (SCC AS) and a Proxy Call Session Control Function, a Serving Call Session Control Function and/or Interrogating Call Session Control function (P/I/S-CSCF). The session transfer controller is typically implemented in the Internet Protocol Multimedia Subsystem (IMS).
The IMS is typically used to control packet switched services offered over the E-UTRA network; control of circuit switched services in a UTRA/GERA network typically involves a mobility controller, such as a Mobility Switching Centre (MSC). The session transfer controller typically communicates with the mobility controller during handover according to a VCC procedure.
User equipment may be equipped with a single radio transceiver, for reasons of economy or for minimising power consumption, so that simultaneous communications with two radio access networks is not possible. In this case the handover protocol typically uses a break-before-make radio connection during handover. Handover procedures known as Single Radio Voice Call Continuity (SR VCC) procedures have been developed for such situations.
In an arrangement, such as where a video or multimedia call is handed over utilizing SRVCC from a packet based network to a circuit based network, the video or multimedia call maybe scaled back to a voice call, i.e. only a voice component of the video or the multimedia call is transferred from the packet based network to the circuit based network.
A circuit switched multimedia service with service change and fallback, SCUDIF, Service Change and UDI/RDI Fallback (SCUDIF) is known and described in 3GPP TS 23.172 V6.3.0 (2005-06): UDI/RDI fallback and service modification, Stage 2. UDI stands for unrestricted digital information, RDI for restricted digital information. UDI/RDI is part of Bearer Capability information passed to/from the network.
SCUDIF allows users to achieve successful call establishment when end to end circuit-switched multimedia call is not possible or when signaling of the feature is not possible in the network. Furthermore, SCUDIF allows the users to swap between a multimedia service and basic speech during an established call.
For video calls initiated in a circuit switched domain, SCUDIF supports the following fallback and change features:
a) fallback to speech during call setup: allow a user to attempt to set up a multimedia call, and try a speech connection if the former doesn't succeed;
b) fallback to the less preferred service (speech or multimedia) during call setup: allow the terminating side via specific settings for this service in the terminal to accept or reject a multimedia call, without interrupting the call setup;
c) fallback to the preferred service (speech or multimedia) or speech during call setup: allow the call setup to proceed with a single service if the transit network does not support the signaling of this functionality;
d) bearer capability (BC) negotiation at the terminating side: allow the terminating side via specific settings for this service in the terminal to turn a speech call (with service change) into a multimedia call and vice-versa;
e) service change: allow a speech call to be turned to multimedia by either of parties, and back to speech, through a successful in call modification procedure;
f) allow any of the users associated with a communications session to reject a multimedia request from the other party while in speech mode and
g) network-initiated service change: the network shall initiate a service-change from multimedia to speech during the active call if a multimedia call can no longer be supported, and if the multimedia call can again be supported at a later point in time, the network may initiate a service change from speech to multimedia.